Brown rice has a short shelf life (approximately 3-6 months) because of hydrolytic and oxidative deterioration of bran lipids. Brown rice lipids are readily hydrolyzed by lipases, both natural to the bran and of microbial origin, that release free fatty acids. Free fatty acids are the precursors of off-flavors and off-odors associated with lipid degradation products generated in subsequent oxidation reactions.
The susceptibility of brown rice to readily becoming rancid has limited the marketing of the kernels, flours, bran, and oil. Bran high in free fatty acids loses its animal feed and food value. At the elevated temperatures experienced during the transport of brown rice from the U.S. to other countries, the oil in the bran is subject to lipolytic hydrolysis. The higher the free fatty acids in the oil the more uneconomical it is to refine. The losses for potentially edible oil during refining are two to three times the free fatty acid content of the oil (Enochian et al, 1981, "Stabilization of rice bran with extruder cookers . . . ", A preliminary analysis of operational and financial feasibility, U.S. Department of Agriculture Marketing Res. Report 1120).
Three approaches have been taken to stabilize brown rice to hydrolysis by lipases: 1) inactivating lipase by subjecting raw or brown rice to moist or dry heat (U.S. Pat. No. 2,585,978 to Van Atta et al, 1952; U.S. Pat. No. 2,992,921 to Bardet et al, 1961; U.S. Pat. No. 4,582,713 to Hirokawa et al, 1986) or to parboiling or precooking processes (U.S. Pat. No. 3,086,867 to Miller, 1963; U.S. Pat. No. 3,959,515 to McCabe, 1976; Sowbhagya and Bhattacharya, 1976, "Lipid autoxidation in rice" J. Food Sci. 41:1018-1023), 2) removing kernel oil which serves as a substrate for lipase by organic solvent extraction [U.S. Pat. No. 2,538,007 to Kester, 1951; U.S. Pat. No. 3,261,690 to Wayne, 1966 (as discussed more fully in "Solvent Extractive Rice Milling" in Rice Chemistry and Technology, D. F. Houston, Editor, Am. Assoc. of Cereal Chemists, St. Paul, Minn.)], and 3) denaturation of lipases, both natural to the bran and of microbial origin, by ethanol extraction, (U.S. patent application Ser. No. 07/557,822 to Champagne et al, 1990). The aforementioned Champagne et al patent application teaches stabilizing brown rice by contacting unmilled brown rice with liquid ethanol.
The present invention is drawn to a highly advantageous and unobvious process for stabilizing unmilled brown rice to lipolytic hydrolysis comprising: contacting unmilled or partially unmilled brown rice with ethanol vapor (i.e. the phrase "ethanol vapor" encompasses either vapor of pure 100% absolute ethanol, or vapor of aqueous mixtures including ethanol) under conditions providing no substantial loss of oil from the unmilled or partially milled brown rice by the ethanol vapor, thereby producing stabilized unmilled or partially milled brown rice and ethanol having rice extracts therein; and separating the stabilized unmilled or partially milled brown rice from the ethanol having rice extracts therein.
Other aspects of the present invention include production by the aforementioned process, of stabilized unmilled brown rice and partially stabilized unmilled brown rice flour having highly advantageous and unobvious properties; e.g. not susceptible to hydrolytic deterioration by lipase, improved storage stability, absence of solvent(s) not generally regarded as safe, having ungelatinized starch as a raw product, having very low bacterial and fungal populations, desirable kernel surface appearance, highly desirable cooking properties, no or minimal loss of oil, minerals, thiamine, protein, dietary fiber and carbohydrates, etc.
Other objects and advantages of the present invention will become readily apparent from the ensuing description.